This invention relates generally to gas tagged nuclear fuel elements and more specifically to a capsule for introducing tag gas into nuclear fuel elements.
During the operation of nuclear reactors there may be a failure of the cladding resulting in release of radioactive fission products to the coolant and ultimately to the environment. Moreover, coolant may enter the failed fuel element and react with the fuel, causing further damage. It is therefore highly desirable to have some means of detecting a cladding failure.
It is also highly desirable to locate, as nearly as possible, the fuel element which has failed.
In liquid metal cooled fast reactors the coolant is blanketed by an inert cover gas. In water-cooled reactors there is a gradual evolution of non-condensable off-gases produced in part by dissociation of the water. Other reactors are cooled by gas, e.g., carbon dioxide or helium. One method detecting a fuel cladding failure in any of these types of reactors is by analysis of the gas.
In order to indicate and identify a fuel element failure, it is known to introduce specific mixtures of different isotopes of, e.g., neon, xenon or krypton into the fuel elements. By using different isotopic mixtures in different subassemblies of the reactor core it is possible to determine the location of the failed fuel element. This is done by subjecting the gas to mass spectrometric analysis. It is then possible to remove the fuel assembly containing the defective fuel element.
The above method is described in U.S. Pat. No. 3,632,470 to Rubin et al., U.S. Pat. No. 3,663,363 to Crouthamel et al., and U.S. Atomic Energy Commission report BNWL-1200-4, pages 4.38 to 4.44. This method has come to be known as "tagging" and the isotopic mixture has come to be known as the "tag gas."
One problem in connection with the tagging has been the manner of introduction of the tag gas into the fuel element. The fuel elements ordinarily contain a fill gas, usually helium, in addition to the nuclear fuel. The helium is normally introduced by placing the fuel tube containing nuclear fuel in a chamber which contains a welding apparatus, evacuating the chamber, backfilling with helium, inserting the end cap into the end of the fuel tube, then welding the end cap to the fuel tube. While the tag gas can be added to the helium during the backfilling step, this is wasteful of the tag gas, which is relatively expensive. Moreover, in some cases, the tag gas may include a radioactive isotope which should not be released into the helium which escapes from the chamber after the welding step. To avoid this problem, the tag gas has been, in one prior art method, enclosed in a capsule fitted with a punch attached to a bimetallic element which causes the punch to penetrate the capsule when a certain temperature is reached in the reactor. See Henault et al., American Nuclear Society Transactions, Vol. 13, page 798 ( 1970). However, since the puncturing does not take place until the fuel is in the reactor, there is no way to be sure that the intended release of the gas from the capsule has actually taken place.
It is an object of this invention to provide a means for releasing tag gas from a capsule within a fuel element which will be positive in operation, which can be operated before the fuel element is placed in the reactor, and which is subject to nondestructive inspection to verify its operation.